This invention relates to fuel cells and, in particular, to a fuel flow control assembly for use of fuel cells.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate between each cell.
Molten carbonate fuel cells (“MCFC”) systems operate by passing a reactant fuel gas through the anode, while oxidizing gas is passed through the cathode. Typical MCFC systems include an anode exhaust gas oxidizer unit downstream from the fuel cell anode, which comprises an oxidation catalyst for oxidizing hydrogen, carbon monoxide and unreacted hydrocarbons in the anode exhaust to produce oxidizing gas suitable for adding to the air or oxidant gas for supply to the fuel cell cathode. In some cases, the oxidizer unit is incorporated into an exhaust gas oxidizer assembly which includes a mixer preceding the oxidizer unit. In this assembly, the anode exhaust gas stream and the cathode supply air or oxidant are first mixed in the mixer and then the mixed gases are fed into the oxidizing unit for oxidizing the exhaust gas via exposure to the oxidation catalyst in the unit. The resultant gas which is rich in oxidant and carbon dioxide is then fed to the cathode of the fuel cell.
MCFC systems typically are heated to temperatures above 700 degrees F. before adding fuel, and therefore require significant start-up time in order to reach these operating temperatures. When the temperatures in the MCFC system are below 1000 degrees F., and in particular, during start-up, or heat-up, periods of the MCFC system operation, the amount of unreacted fuel in the form of methane present in the anode oxidizer assembly exhaust is significantly greater than after the MCFC system reaches its operating temperature. The presence of high methane concentration in the anode exhaust often results in incomplete oxidation of the anode exhaust gas in the oxidizing unit, allowing methane gas to slip through the oxidizing catalyst unreacted. Below 1000° F., reaction of methane and other hydrocarbons in the oxidizer is not assured. In addition, high methane concentrations and accumulation of the methane gas in the anode gas oxidizer assembly exhaust gas can potentially result in a formation of an ignitable gas mixture of significant volume, creating undesired conditions in the MCFC system.
To avoid the formation of the above-mentioned mixture in the MCFC system and the slipping of excessive unreacted methane gas through the oxidizing catalyst, the flow rate of the fuel flowing into the MCFC system needs to be accurately controlled. A common method of controlling the fuel flow rate is to employ a gas analyzer to detect excess methane in the anode gas oxidizer assembly exhaust and to control the fuel flow rate based on the detection by the analyzer so as to prevent the accumulation of methane in the anode gas oxidizer assembly exhaust and to prevent the formation of a potentially ignitable mixture. The fuel flow rate is commonly controlled using two valves downstream from the fuel supply and upstream from the fuel cell anode, wherein a first valve is sized to allow a low flow of the fuel gas therethrough and a second valve is sized to allow medium to high flow of fuel gas therethrough. Alternatively, the fuel flow rate has been controlled by varying the inlet pressure of the fuel gas flowing into the MCFC system using an automated pressure control valve. The latter method of controlling the fuel flow rate is often accomplished by setting the fuel pressure to a minimum pressure which allows the fuel flow required for full load operation of the MCFC system.
These commonly employed methods of controlling the fuel flow rate to the fuel cell anode require additional control valves and piping, as well as complex logic for controlling these valves and are susceptible to instrument or valve failure. Accordingly, the system manufacturing and operating costs are significantly increased. Moreover, the common practice of setting the fuel pressure to a minimum required pressure often results in insufficient fuel being provided to the MCFC system operating at full load, particularly when the MCFC system includes restacked fuel cell units which produce more power and require a higher fuel flow rate. Such practice, therefore, affects the operating efficiency of the MCFC system.
Accordingly, a system which effectively prevents the slipping of excess methane gas through the oxidizing catalyst and avoids formation of the undesired conditions in the MCFC system is needed. In particular, an assembly and a method for controlling a fuel flow rate to the MCFC system which is accurate, more cost-efficient and less susceptible to failure are needed. Moreover, an assembly and a method of controlling the fuel flow rate which do not adversely affect the operating efficiency of the MCFC system are also desired.
It is therefore an object of the present invention to provide a fuel cell system which is adapted such that during start-up or heat-up of the system fuel flow is controlled to avoid undesired igniting conditions of the gases of the system.
It is a further object of the invention to provide the above fuel cell system in a manner which preserves the operating efficiency of the system.